Use of thermal insulating fluids in wells

ABSTRACT

A process of thermally insulating the borehole of a well by providing in the annular space of the well between the wall of the borehole and a well conduit a suspension of an ultra finely divided silica in a low thermal conductivity oil.

United States Patent Culter Aug. 12, 1975 .[54] USE OF THERMALINSULATING FLUIDS IN 3,634,563 1/ 1972 Asbury et al. 252/62 X WELLS3,642,624 2 1972 Howland et al 252/62 x 3,700,050 10/1972 Miles 166/DIG.I X [75] Inventor: John D. Culter, Ashevllle, N.C. 3,719,601 3/1973Jacocks 166/302 X 3,722,591 3/1973 Maxson l66/DlG. l X [73] Asslgnee'cfmi'nemal Ponca 3,831,678 8/1974 Mondshine 166/DIG. 1 x

City, Okla.

[22] Flled: 1974 Primary ExaminerStephen J. Novosad [21] Appl. No.:455,446

52 US. Cl. 166/302; 166/303; l66/DIG. 1 [571 ABSTRACT 2 2; 5 2 A processof thermally insulating the borehole of a 1 o earc l well by providingin the annular space of the well between the wall of the borehole and awell conduit a suspension of an ultra finely divided silica in a low[56] References cued thermal conductivity oil.

UNITED STATES PATENTS 3,618,680 11/1971 Ellard et a1. l66/DIG. l X 3Claims, N0 Drawings USE OF THERMAL INSULATING FLUIDS IN WELLS BACKGROUNDOF THE INVENTION 1. Field of the Invention I This invention relates to amethod of thermally insulating the annular space in a well surrounding awell conduit to cut down heat transfer between the conduit and thecasing or borehole wall from fluids passing through the conduit. Highheat transfer can result in damage to casing, the bond between thecasing and the cement behind it or to the surrounding formation itself.

2. Description of the Prior Art In various well operations heat may betransferred from relatively hot fluids flowing through well conduitssuch as tubing outwardly to the relatively cold casing or borehole wallof an uncased well. Similarly, heat transfer may be from a relativelyhot casing or borehole wall to a relatively cool fluid flowing through awell conduit.

In thermal stimulation operations steam or hot water is injected downthe tubing of a cased Welland into a subterranean formation containinghigh viscosity oil. The hot fluids raise the temperature of and decreasethe viscosity of the oil which is then produced via the same or adifferent well. The temperature of the injected steam or hot water isquite high. Considerable heat is conducted outwardly through the tubing,the annular space between the tubing and the casing and through thecasing which is generally cemented to the borehole wall. This oftencauses stresses in the casing due to thermal expansion which may resultin failure of the casing such as by buckling or rupture of thec'asingcement bond.

In wells drilled in arctic environments, drilling fluids or fluidsproduced from the well following completion are at a higher temperaturethan the permafrost region through which the well is drilled. It isdesirable to cut down heat transfer from the fluids flowing through thetubing or drill string to the permafrost formation to minimize meltingof the permafrost which can result in loss of stability of the casing.

In wells producing fluids from relatively hot formations, it may bedesirable to maintain such fluids at a relatively high temperatureduring their passage up a well conduit to the surface, as when coolingof these fluids may cause the paraffin constituents therein to come outof solution, deposit on the conduit wall and possibly plug the same.

A wide variety of materials have been suggested as insulating fluids forthe annulus between a well conduit and the surrounding casing orborehole wall to cut down heat transfer between the conduit to itssurrounding environment. Such materials include oil thickened withvarious thickening agents such as soaps, vermiculate, asbestos,magnesium silicate and the like. However, there still exists a need forsuch an insulating fluid which possesses the desired insulatingproperties and is able to withstand the high temperature of steam orsimilar fluids without deterioration.

OBJECTS OF THE INVENTION It is an object of this invention to provide aprocess for thermally insulating the annular space between a wellconduit and the surrounding environment which may be a casing or aborehole wall.

It is a further object to provide such a process which providesimproved-insulation under conditions where eitherthe well conduit or itssurrounding environment are at a high temperature.

It is .a still further object to'provide such a process wherein theinsulating medium is stable at high temperatures.

Other objects, advantages and features of this invention will becomeapparent from the following description and claims. 7

BRIEF SUMMARY OF THE INVENTION This invention involves a process ofthermallyinsulating the casing or borehole of a well and its'environmentfrom a fluid passing through a well conduit such as tubing or drillstring by providing in the annular space between the casing of boreholeand the well conduit a thermal insulating fluid comprising an oil havinglow thermal conductivity containing an ultra finely divided silicasuspended therein DESCRIPTION OF THE PREFERRED EMBODIMENT The thermalinsulating fluid used inthe process of this invention is a suspension ofan ultra finely divided silica in an oil having a low thermalconductivity/By ultra finely divided silica is meant silica having aparticle size of less than about 0.1 micron. Such pyrogenic silicas canbe prepared at high temperatures of about 1100C by the vapor phasehydrolysis of silicon tetrachloride. The silica is collected from thegaseous phase. Larger particle size silicas also form a thermalinsulating fluid when mixed with the oils of this invention, but theresulting suspensions tends to settle with time especially inenvironments of elevated temperature resulting in unsatisfactory thermalinsulating properties in the upper regions of the fluid column.

The oil having a low thermal activity can be selected from such oils aspale oils, fuel oils and bright stocks. Specific examples includefurfural extracted pale oil, No. 6 fuel oil, viscosity index brightstock (a paraffinic mineral oil) and pale oil.

The concentration of ultra finely divided silica in the low thermalconductivity oil can range from 0.15 pounds per gallon to 1.0 pounds pergallon. At concentrations of less than 0.15 pounds per gallon thecomposition has insufficient gel strength, i.e., is too fluid to providesufficient insulation properties. At concentrations above about 1.0pounds per gallon the composition is a gel which is so viscous that itbecomes difficult to pump and handle. The mixing may be carried out byany convenient stirring device such as a bladed stirrer or the like. Thecomposition may be used as an insulating fluid over a wide temperaturerange from protecting permafrost formations to steam injection wellswhere the temperature of the steam approaches 700F.

A suspension was prepared of ultra finely divided silica having aparticle size range of from 0.015 to 0.02 micron in furfural extractedpale oil. A concentration of 0.32 pound silica per gallon pale oil wasused. The gel strength of the resulting mixture was 2.5 pounds per 100square feet at room temperature and 2.0 pounds per 100 square feet at F.Thus the gel strength did not change appreciably with temperature.

An annular test cell was prepared by welding together at each and withannular discs, concentric 32 inch long sections of a 2"/s inch diametertubing and a 7 inch diameter casing. Access ports to the annular spacebetween the tubing and the casing were provided in the casing near eachend thereof. Thermocouples were located at selected positions along thetubing and casing axis and were used to monitor the temperature of each.A heating element with guard heaters at either end was centered insidethe tubing to provide a uniform source of heat from the surface of thetubing. Two rows of copper tubing with holes drilled at uniformintervals were located at both ends of the casing to help end effects.Air was used as the coolant. The annular space of the test cell wasfilled with a thermal insulating fluid prepared by mixing an ultrafinely divided silica into 95 viscosity index bright stock with alaboratory stirrer. The pyrogenic silica used had a particle size rangeof 0.015 to 0.020 micron, a surface area of 175 to 200 square meters pergram and a specific gravity of 2.1. The silica was used at aconcentration of 0.24 pounds per gallon of bright stock. Heat wasapplied to the tubing at the rate of 277 BTU/hr-ft F. After 68 hours ofheating the average tubing temperature was 521F. and the average casingtemperature was 160F. The apparent thermal conductivity of the thermalinsulating fluid was 0.095 BTU/hr-ft F. Heating was continued. After 74hours the heat input rate was increased to obtain an average tubingtemperature of 650F. and an average casing temperature of 170F. Theapparent thermal conductivity of the thermal insulating fluid was 0.106BTU/hr-ft F. The test was terminated after 164 hours of heating. Samplesof the thermal insulating fluid were withdrawn from the top, middle andbottom portions of the annulus, an ash determination was made todetermine the amount of silica in each sample. The top sample contained3.46 weight percent ash. The middle sample contained 3.34 weight percentsilica. The bottom sample contained 3.24 weight percent ash. Theseresults indicate that little segregation of the silica had occurred inthe fluid during the heating period. The above temperature resultsshowed that the fluid provided excellent protection to the test casing.

The thermal insulating fluid can be used in a well by mixing together atthe surface the ultra finely divided silica and the low thermalconductivity oil. The resulting suspension is then pumped into theannular space between the tubing and the borehole wall or casingfollowing well known placement techniques. The annulus is generallyclosed off near the bottom of the tubing by a packer positioned betweenthe tubing and the borehole wall or casing.

It is to be understood that the above description is only illustrativeof the various ways in which the process of this invention may becarried out. Other modifications within the scope of the invention willoccur to those skilled in the art.

What is claimed is:

l. A method of thermally insulating an annular space between theborehole of a well and a conduit therein comprising providing in saidannular space a suspension comprised of ultra finely divided silicahaving a particle size of less than 0.1 micron in an oil having lowthermal conductivity, said silica being present in an amount of from0.15 to 1.0 pounds per gallon.

2. The method of claim 1 wherein the ultra finely divided silica has aparticle size range of from 0.015 to 0.02 microns.

3. The method of claim 1 wherein the oil having low thermal conductivityis-selected from a group consisting of pale oil, fuel oil and brightstock.

1. A METHOD OF THERMALLY INSULATING AN ANNULAR SPACE BETWEEN THEBOREHOLE OF A WELL AND A CONDUIT THEREIN COMPRISING PROVIDING IN SAIDANNULAR SPACE A SUSPENSION COMPRISED OF ULTRA FINELY DIVIDED SILICAHAVING A PARTICLE SIZE OF LESS THAN 0.1 MICRON IN AN OIL HAVING LOWTHERMAL CONDUCTIVELY, SAID SILICA BEING PRESENT IN AN AMOUNT OF FROM0.15 TO 1.0 POUNDS PER GALLON.
 2. The method of claim 1 wherein theultra finely divided silica has a particle size range of from 0.015 to0.02 microns.
 3. The method of claim 1 wherein the oil having lowthermal conductivity is selected from a group consisting of pale oil,fuel oil and bright stock.